Metallic compositions which that are known to be capable of undergoing a reversible transformation from the austenitic state to the martensitic state include unalloyed metals but this phenomenon is most commonly exhibited by alloys. Such alloys include, for example, those disclosed in U.S. Pat. Nos. 3,012,882; 3,174,851; 3,351,463; 3,567,523; 3,753,700; and 3,759,552, Belgian Pat. No. 703,649 and in British patent application Nos. 22372/69, 55481/69, 55482/69, 55959/69 and 53734/70 (now British Pat. Nos. 1,315,652; 1,315,653; 1,346,046 and 1,346,047) in the name of the Fulmer Research Institute. The disclosure of each of the aforementioned patents and applications is incorporated herein by reference.
Such alloys are also disclosed in NASA Publication SP5110, "55-Nitinol-the alloy with a memory, etc." (U.S. Government Printing Office, Washington, D.C., 1972), N. Nakanishi et al, Scripta Metallurgica 5, 433-440 (Pergamon Press 1971), the disclosures of which are likewise incorporated herein by reference.
These, and other alloys, have in common the feature of undergoing a shear transformation on cooling from a relatively high temperature (austenitic) state to a low temperature (or martensitic) state. If an article made of such an alloy is deformed when in its martensitic state it will remain so deformed. If it is heated to return it to a temperature at which it is austenitic, it will tend to return to its undeformed state. The transition from one state to the other, in each direction, takes place over a temperature range. The temperature at which martensite starts to form on cooling is designated M.sub.s while the temperature at which this process is complete is designated M.sub.f, each of these temperatures being those achieved at high, e.g., 100.degree. C per minute, rates of change of temperature of the sample. Similarly, the temperature of the beginning and end of the transformation to austenite are designated A.sub.s and A.sub.f, respectively. Generally, M.sub.f is a lower temperature than A.sub.s, M.sub.s is a lower temperature than A.sub.f, and M.sub.s can be lower, equal to or higher than A.sub.s, for a given alloy depending on composition and thermomechanical history. The transformation from one form to the other may be followed by measuring one of a number of physical properties of the material in addition to the reversal of deformation described above, for example, its electrical resistivity, which shows an anomaly as the transformations take place. If graphs of resistivity-v-temperature or strain-v-temperature are plotted, a line joining the points M.sub.s, M.sub.f, A.sub.s, A.sub.f and back to M.sub.s forms a loop termed the hysteresis loop (see FIG. 3). For many materials M.sub.s and A.sub.s are at approximately the same temperature.
One particularly useful alloy possessing heat recoverability or shape memory is the intermetallic compound TiNi, as described in U.S. Pat. No. 3,174,851. The temperature at which deformed objects of the alloys return to their original shape depends on the alloy composition as disclosed in British Pat. No. 1,202,404 and U.S. Pat. No. 3,753,700, e.g., the recovery of original shape can be made to occur below, at, or above room temperature.
In certain commercial applications employing heat recoverable alloys, it is desirable that A.sub.s be at a higher temperature than M.sub.s, for the following reason. Many articles constructed from such alloys are provided to users in a deformed condition and thus in the martensitic state. For example, couplings for hydraulic components, as disclosed in U.S. patent applications Ser. Nos. 852,722 filed Aug. 25, 1969 and 51809 filed July 2, 1970 (British Pat. Nos. 1,327,441 and 1,327,442), are sold in a deformed (i.e., an expanded) state. The customer places the expanded coupling over the components (for example, the ends of hydraulic pipe lines) to be joined and raises the temperature of the coupling. As its temperature reaches the austenitic transformation range, the coupling returns, or attempts to return, to its original configuration, and shrinks onto the components to be joined. Because it is necessary that the coupling remain in its austenitic state during use (for example, to avoid stress relaxation which occurs during the martensitic transformation and because its mechanical properties are superior in the austenitic state), the M.sub.s of the material is chosen so as to be below the lowest temperature which it may possibly reach in service. Thus, after recovery, during service the material will remain at all times in the austenitic state. For this reason, once deformed it has to be kept in, for example, liquid nitrogen until it is used. If, however, the A.sub.s (A.sub.s, as used herein, means that temperature which marks the beginning of a continuous sigmoidal transition as plotted on a strain vs. temperature graph, to the austenitic state of all the martensite capable of undergoing that transformation) could be raised, if only temporarily for one heating cycle, without a corresponding rise in the M.sub.s then the expanded coupling could be maintained at a higher and more convenient temperature. The advantage this would provide is an obvious one. For example, if the A.sub.s of the alloy from which it is made could be raised sufficiently to allow the coupling to be handled at ambient temperature without recovery occuring, it would be possible to avoid the problems and expense associated with prolonged storage of the heat recoverable coupling that must be kept in liquid nitrogen after deformation.
In our co-pending and commonly assigned U.S. application "Heat Treating Method," Ser. No. 550,847, also filed on even date herewith, as a C.I.P. of application Ser. No. 417,067, filed Nov. 19, 1973, the disclosures of both of which are incorporated by reference, we have described a method by which the A.sub.s of certain metallic compositions can be raised for one heating cycle. This method comprises first lowering the temperature of the composition from that at which it exists in the austenitic state to below its M.sub.f temperature. Then the composition is heated to a temperature at which normally it would exist wholly in the austenitic state, i.e., above the A.sub.f temperature. However, the transformation from martensite to austenite does not occur if the heating rate selected is a "slow" one. The definition of a "slow" heating rate is fully set forth in said co-pending application. Suffice it to say that it can vary depending upon the nature of the metallic composition but is easily determined by one skilled in the art having the benefit of said application.
If the composition is cooled after slow heating is complete and subsequently reheated at a rapid rate, it does not begin to undergo a martensite to austenite transformation until the approximate temperature at which slow heating was terminated is reached. More importantly, if an article was made from the composition and deformed while in the martensite state either prior to, or after, slow heating is terminated, it will not begin to undergo recovery to the form in which it existed in the austenitic state until it reaches approximately the temperature at which slow heating was suspended. We refer to this process as "thermal preconditioning."
In another co-pending U.S. application, "Mechanical Preconditioning Method," Ser. No. 550,555 filed on even date herewith and now U.S. Pat. No. 4,036,669, the disclosure of which is incorporated by reference, we have described yet another method by which the A.sub.s temperature of metallic compositions can be elevated. That method comprises holding the composition in a deformed configuration at a temperature above its normal A.sub.s -A.sub.f range for a length of time sufficient to cause a portion of the deformation to be retained when the constraint is removed. The amount of deformation retained is a function of the temperature at which the composition is held and the duration of the holding step.
The composition can be deformed while in the austenitic state. Typically, however, this requires a great deal of force. Accordingly, it is preferred to deform the composition while it is in a more workable condition that occurs near, within or below the M.sub.s -M.sub.f range and then to raise its temperature while restrained to the desired holding temperature.
By analogy to "thermal preconditioning, " this method is referred to as "mechanical preconditioning." An article preconditioned in this way when heated at a fast rate will recover a portion of the retained strain.
As a result of our discoveries, it has been possible to prepare heat recoverable articles having an elevated A.sub.s temperature. Frequently, however, metallic compositions which have been transformed into the martensitic state exhibit a tendency to lose all or a portion of their ability to revert back to austenite when heated through the A.sub.s -A.sub.f range. In other instances, metallic compositions do not favorably respond to either thermal or mechanical preconditioning procedures for elevating the A.sub.s temperature. Obviously, it would be of great advantage to have available a method of inhibiting the loss of these desirable properties.
Accordingly, it is an object of this invention to provide a method by which the loss of martensite-austenite reversibility in metallic compositions is inhibited. It is yet another object to provide a method by which metallic compositions can be rendered more responsive to methods of imparting an elevated A.sub.s temperature. Yet another object of this invention is to provide metallic compositions having a reduced tendency to lose martensite-austenite reversibility and more responses to methods of imparting an elevated A.sub.s.